This invention relates to a middle value selection circuit. More particularly, the present invention relates to a middle value selection circuit which is particularly suitable for use in a multiple redundant control apparatus for various systems for which high reliability is a requisite.
To improve the safety of a nuclear power plant, a triple redundant control apparatus is used for an especially important control system. The middle value of the output signals from the three processing units is selected by one middle value selection circuit and is used as a control signal for controlling an actuator that is disposed in a control valve or the like.
The triple redundant control apparatus of the type described above includes the middle value selection circuit or circuits in order to select the middle value of the input and output signals, as described above. An example of the middle value selection circuit is disclosed in Japanese Patent Laid-Open No. 11638/1975 and Japanese Utility Model Laid-Open No. 117001/1980.
The middle value selection circuit must select a signal which is the most approximate to a true value from the three signals; hence, it is necessary to reduce the error of the output signals as much as possible.